System and Method for Device-to-Device Communications

ABSTRACT

A method for device-to-device (D2D) communication includes determining that a first user equipment (UE) is out-of-coverage, the first UE having been previously in-coverage on a first cell. The method also includes starting a timer upon determining that the first UE is out-of-coverage and determining whether the first UE has returned to be in-coverage after starting the timer. Additionally, the method includes determining whether the timer has expired and communicating, by the first UE directly with a second UE, using out-of-coverage resources from the first cell when the timer has not expired and the first UE has not returned to be in-coverage.

This application claims the benefit of U.S. Provisional Application Ser.No. 61/927,347 filed on Jan. 14, 2014, and entitled “System and Methodfor Device-to-Device Communication Protocol,” which application ishereby incorporated herein by reference.

TECHNICAL FIELD

The present invention relates to a system and method for wirelesscommunications, and, in particular, to a system and method fordevice-to-device communications.

BACKGROUND

Device-to-device (D2D) technology is expanding because of its ability tooffer new services, improve system throughput, and offer a better userexperience. D2D technologies include discovery and communication. Indiscovery, a user equipment (UE) attempts to discover neighboring UEs,either on its own or directed by an enhanced Node B (eNB). Incommunication, one UE directly communicates with another UE without thedata transiting through the eNB.

SUMMARY

An embodiment method for device-to-device (D2D) communication includesdetermining that a first user equipment (UE) is out-of-coverage, thefirst UE having been previously in-coverage on a first cell. The methodalso includes starting a timer upon determining that the first UE isout-of-coverage and determining whether the first UE has returned to bein-coverage after starting the timer. Additionally, the method includesdetermining whether the timer has expired and communicating, by thefirst UE directly with a second UE, using out-of-coverage resources fromthe first cell when the timer has not expired and the first UE has notreturned to be in-coverage.

An embodiment method for device-to-device (D2D) communications includescomputing an in-coverage threshold (ICT) parameter and determiningwhether to perform contention-based resource allocation orscheduling-based resource allocation in accordance with the ICTparameter. The method also includes performing contention-based resourceallocation to produce allocated resources when determining to performcontention-based resource allocation and performing scheduling-basedresource allocation to produce the allocated resources when determiningto perform scheduling-based resource allocation. Additionally, themethod includes communicating, by a first user equipment (UE) directlywith a second UE using the allocated resources.

An embodiment first user equipment (UE) includes a processor and anon-transitory computer readable storage medium storing programming forexecution by the processor. The programming includes instructions todetermine that the first UE is out-of-coverage, the first UE having beenpreviously in-coverage on a first cell. The programming also includesinstructions to start a timer upon determining that the first UE isout-of-coverage and determine whether the first UE has returned to bein-coverage after starting the timer. Additionally, the programmingincludes instructions to determine whether the timer has expired andcommunicate, directly with a second UE, using out-of-coverage resourcesfrom the first cell when the timer has not expired and the first UE hasnot returned to be in-coverage.

The foregoing has outlined rather broadly the features of an embodimentof the present invention in order that the detailed description of theinvention that follows may be better understood. Additional features andadvantages of embodiments of the invention will be describedhereinafter, which form the subject of the claims of the invention. Itshould be appreciated by those skilled in the art that the conceptionand specific embodiments disclosed may be readily utilized as a basisfor modifying or designing other structures or processes for carryingout the same purposes of the present invention. It should also berealized by those skilled in the art that such equivalent constructionsdo not depart from the spirit and scope of the invention as set forth inthe appended claims.

BRIEF DESCRIPTION OF THE DRAWINGS

For a more complete understanding of the present invention, and theadvantages thereof, reference is now made to the following descriptionstaken in conjunction with the accompanying drawing, in which:

FIG. 1 illustrates a diagram of a wireless network for communicatingdata;

FIG. 2 illustrates coverage scenarios for user equipments (UEs);

FIG. 3 illustrates an idle mode state diagram;

FIGS. 4A-B illustrate an idle mode cell selection and reselection statediagram;

FIG. 5 illustrates a flowchart for an embodiment method ofdevice-to-device (D2D) resource allocation;

FIG. 6 illustrates an embodiment state diagram for D2D resourceallocation;

FIG. 7 illustrates another embodiment state diagram for D2D resourceallocation;

FIG. 8 illustrates a flowchart for another embodiment method of D2Dresource allocation;

FIG. 9 illustrates a flowchart for an additional embodiment method ofD2D resource allocation; and

FIG. 10 illustrates a block diagram of an embodiment computer system.

Corresponding numerals and symbols in the different figures generallyrefer to corresponding parts unless otherwise indicated. The figures aredrawn to clearly illustrate the relevant aspects of the embodiments andare not necessarily drawn to scale.

DETAILED DESCRIPTION OF ILLUSTRATIVE EMBODIMENTS

It should be understood at the outset that although an illustrativeimplementation of one or more embodiments are provided below, thedisclosed systems and/or methods may be implemented using any number oftechniques, whether currently known or in existence. The disclosureshould in no way be limited to the illustrative implementations,drawings, and techniques illustrated below, including the exemplarydesigns and implementations illustrated and described herein, but may bemodified within the scope of the appended claims along with their fullscope of equivalents.

Two device-to-device (D2D) technologies are discovery and communication.In discovery, a user equipment (UE) attempts to discover neighboringUEs, either on its own or directed by a communications controller, suchas an enhanced Node B (eNB). In communication, one UE directlycommunicates with another UE without the data transiting through theeNB. One issue in D2D communications is resource allocation. Resourceallocation may be performed for UEs engaged in D2D communication,whether they are in network coverage or out of network coverage.

Resource allocation may be contention-based or scheduling based. Inscheduling-based resource allocation, an eNB determines the resourceallocation, while in contention-based resource allocation, the UEscontend (compete) for the use of the resources.

FIG. 1 illustrates network 100 for communicating data. Network 100includes communications controller 102 having a coverage area 106, aplurality of UEs, including UE 104 and UE 105, and backhaul network 108.Two UEs are depicted, but many more may be present. Communicationscontroller 102 may be any component capable of providing wireless accessby establishing uplink (dashed line) and/or downlink (dotted line)connections with UE 104 and UE 105, such as a base station, a NodeB, aneNB, an access point, a picocell, a femtocell, and other wirelesslyenabled devices. There may be D2D communication between UE 104 and UE105. UE 104 and UE 105 may be any component capable of establishing awireless connection with communications controller 102, such as cellphones, smart phones, tablets, sensors, etc. Backhaul network 108 may beany component or collection of components that allow data to beexchanged between communications controller 102 and a remote end. Insome embodiments, the network 100 may include various other wirelessdevices, such as relays, etc.

The locations of UEs relative to the eNB may affect the coverage. FIG. 2illustrates system 110 with eNB 111. In in-network coverage area 112,UEs, such as UE 118 and UE 120, are able to transmit messages to eNB 111and receive messages from eNB 111. In out-of-network coverage area 116,UEs, such as UE 126 and UE 128, cannot communicate with eNB 111 at all.On the other hand, in area 114, also known as partial coverage area oredge of network, UEs, such as UE 122 and UE 124 can receive messagesfrom eNB 111 but cannot transmit messages to eNB 111. In another partialcoverage scenario, of two UEs performing D2D communication, one UE islocated in-network coverage and the other UE is out-of-network coverageor has partial coverage.

Two approaches to D2D are a device-centric approach and anetwork-managed approach. In a device-central approach, thefunctionality for D2D resides mostly on the UEs with minimal networkinvolvement, and direct communications between devices forms an overlayon top of the cellular network. Functions such as resource allocationand management are performed in an ad-hoc manner between the UEs withoutnetwork oversight or management, limiting the potential performancegains from D2D and introducing challenges for supporting some functionsof the cellular network. Some challenges include the security of userdata and protecting user identity and/or location from being discoveredby unauthorized parties, charging for D2D and proximity as a service,supporting lawful intercept of communications, and the scalability todifferent deployment scenarios and device densities.

In a network-managed approach, D2D complements and enhances thecapabilities of the network for more efficient utilization of radioresources for proximate communications. The network may constantlysupervise the usage of direct versus infrastructure routing of user datato achieve efficient utilization of resources, minimize interference,and benefit the network. Features of the cellular network may beextended to support D2D with a network-managed approach.

Two resource management schemes include scheduling-based resourceallocation and contention-based resource allocation. In scheduling-basedresource allocation, a central controller, such as an eNB, allocatesresources to each UE. The process may be similar to how a cellularcommunication is scheduled. The UE may us a random access channel (RACH)or another contention-based channel to request resources. The centralcontroller allocates resources and transmits the resource allocation ona control channel which may be similar to a physical downlink controlchannel (PDCCH) or an enhanced PDCCH (ePDCCH).

In contention-based resource allocation, a central controller is notused. Each UE attempts to obtain the channel and schedules its ownresources for its transmission. A resource allocation protocol such ascarrier sensing multiple access (CSMA) may be used.

A UE in idle mode may perform processes such as public land mobilenetwork (PLMN) selection, cell selection and reselection, locationregistration, and support for manual closed subscriber group (CSG)selection. FIG. 3 illustrates state diagram 130 for idle mode processes.When a UE is switched on, PLMN selection is performed by the non-accessstratum (NAS) in the PLMN selection state 132. For the selected PLMN,associated radio access technology (RAT)(s) may be selected. The NASprovides a list of equivalent PLMNs. After PLMN selection, the UEproceeds to cell selection and reselection state 136.

The access stratum (AS) uses the list of equivalent PLMNs in cellselection and reselection state 136. The UE searches for a suitable cellfor the selected PLMN, and chooses the selected cell to provideavailable services. Also, the UE tunes to its control channel. Thisselection is known as camping on the cell.

When the registration area changes, the UE registers its presence usinga NAS registration procedure in the tracking area of the chosen cell inlocation registration state 138. As a result of a successful locationregistration, the selected PLMN becomes the registered PLMN, and the UEproceeds to the cell selection and reselection state 136. When thelocation registration is rejected by the network, the UE may return tothe PLMN selection state 132, and select another PLMN when applicable.

When the UE finds a more suitable cell in accordance with the cellselection criteria, the UE reselects that cell and camps on it. When thenew cell does not belong to at least one tracking area to which the UEis registered, location registration is performed.

The UE may search for higher priority PLMNs at regular intervals. The UEsearches for a suitable cell when another PLMN has been selected by NAS.

A search of CSGs may be triggered by NAS in support CSG selection state134.

When the UE loses coverage from the registered PLMN, either a new PLMNis selected automatically in automatic mode, or an indication ofavailable PLMNs is given to the user, so that a manual selection may beperformed in manual mode.

In multimedia broadcast multimedia service (MBMS) frequencyprioritization state 140 is performed and MBMS frequencies areprioritized.

Camping on a cell in idle mode entails a variety of functions. Itfacilitates the UE receiving system information from the PLMN. Also,when a registered UE wants to establish a radio resource control (RRC)connection, it may do so by initially accessing the network on thecontrol channel of the cell on which it is camped. When the PLMNreceives a call for the registered UE, it usually knows the set oftracking areas in which the UE is camped. It may then send a pagingmessage for the UE on the control channels of all the cells in this setof tracking areas. The UE then receives the paging message because it istuned to the control channel of a cell in one of the registered trackingareas, and the UE may respond on that control channel.

When the UE is unable to find a suitable cell to camp on, or when thelocation registration fails, it may attempt to camp on a cellirrespective of the PLMN identity, and enter a limited service state.

FIGS. 4A-B illustrate state diagram 150 for idle cell selection andreselection. When a new PLMN selection is performed, the UE goes tostate 152.

Initial cell selection is performed in initial cell selection state 156.Prior knowledge of which radio frequency (RF) channels are evolveduniversal mode telecommunications system (UMTS) terrestrial radio accessnetwork (E-UTRA) carriers is not required. The UE scans the RF channelsin the E-UTRA bands to find a suitable cell. On each carrier frequency,the UE only searches for the strongest cell. When a suitable cell isfound, it is selected, and the UE proceeds to camped normally state 162.When no suitable cell is found, the UE proceeds to any cell selectionstate 168 via state 174.

In stored information cell selection state 154, stored information ofcarrier frequencies and optionally information on cell parameters frompreviously received measurement control information elements or frompreviously detected cells is used. When the UE has found a suitablecell, the UE selects it, and proceeds to camped normally state 162. Whenno suitable cell is found, the UE proceeds to initial cell selectionstate 156.

In camped normally state 162, the UE is camped on an eNB. While the UEis in the camped normally state, it has only dedicated priorities, andthe UE considers the current frequency to be the lowest priorityfrequency. When the UE is in camped normally state 162, and it leavesidle mode, it goes to connected mode 164. When the UE receives atrigger, it proceeds to cell reselection evaluation process state 166.When NAS indicates that the registration on the selected PLMN has beenrejected, it proceeds to any cell selection state 168 through state 174.

In connected mode 164, the UE operates in connected mode. The UEcommunicates with the eNB.

Then, when the UE returns to idle mode, it proceeds to cell selectionwhen leaving idle mode state 160. When a suitable cell is found, the UEproceeds to camped normally state 162. When no suitable cell is found,the UE proceeds to information cell selection state 154. When theconnected mode fails, it proceeds to cell selection when connected modefailed state 163.

In cell selection when connected mode failed state 163, the UE performscell selection. When the UE is able to connect to a cell, either a newcell or the cell it was previously connected to, it returns to connectedmode 164. On the other hand, when the UE is not able to connect to acell, it proceeds to cell selection when leaving connected mode state160.

In cell reselection evaluation process state 166, cell reselectionevaluation is performed. When a suitable cell is found, either a newcell or the cell the UE was previously connected to, the UE returns tocamped normally state 162. On the other hand, when no suitable cell isfound, the UE proceeds to any cell selection state 168 via state 174.

In any cell selection state 168, the UE attempts to find an acceptablecell of any PLMN to camp on, trying all RATs supported by the UE andsearching first for a high quality cell. When universal subscriberidentity module (USIM) is inserted, the UE proceeds to state 152. On theother hand, when an acceptable cell if found, the UE proceeds to campedon any cell state 172.

In camped on any cell state 172, the UE selects and monitors theindicated paging channels of the cell and monitors relevant systeminformation. Also, the UE performs measurements for the cell reselectionevaluation procedure. Additionally, the UE proceeds to the cellreselection evaluation process state 180 on UE internal triggers or wheninformation on the broadcast control channel (BCCH) used for the cellreselection evaluation procedure has been modified. The UE regularlyattempts to find a suitable cell trying all frequencies of all RATSsupported by the UE. When a suitable cell is found, the UE proceeds tocamped normally state 162 via state 158. When the UE supports voiceservices and the current cell does not support emergency calls, the UEperforms cell selection/reselection to an acceptable cell of anysupported RAT regardless of priorities provided in system informationfrom the current cell when no suitable cell is found. When the UE leavesidle mode, it proceeds to connected mode emergency calls only state 178.

In cell reselection evaluation process state 180, reselection prioritiesare handled. When an acceptable cell is found, the UE returns to campedon any cell state 172. On the other hand, when no acceptable cell isfound, the UE proceeds to any cell selection state 168.

In connected mode emergency calls only state 178, the UE only takesemergency calls. When the UE returns to idle mode, it proceeds to cellselection when leaving connected mode state 170.

In cell selection when leaving connected mode state 170, the UE performscell selection. When an acceptable cell is found, the UE proceeds tocamped on any cell state 172. When no acceptable cell is found, the UEproceeds to any cell selection state 168.

Embodiments include both in-network and out-of network coverage. When aUE is in-coverage, it receives scheduled resources for D2D communicationtransmissions from the network. When the UE is in out-of-networkcoverage, it uses a contention based scheme to select air interfaceresources for D2D transmission. A UE is considered to be in coveragewhen the UE is in an connected mode, camped in a suitable cell, is inconnected mode, or satisfies an in-coverage threshold. On the otherhand, a UE is in out-of-network coverage when in the UE isout-of-coverage. A UE may be considered to be out of coverage when itwas previously in coverage and has lost coverage, is in cell selectionwhen connected mode failed state, camped on any cell, or does notsatisfy the in-coverage threshold.

An in-coverage threshold (ICT) is:

ICT=Q _(rxlevmeas)−(Q _(rxlevmin) +Q _(D2Doffset))−Pcompensation

where ICT is the in-coverage threshold in decibels (dB), Q_(rxlevmeas)is the measured reference signal received power (RSRP) from the servingor camping cell, Q_(rxlevmin) is the minimum required reception level inthe cell in dBm, Q_(D2Doffset) is the offset used for D2D communicationsdeterminations, and Pcompensation is the maximum ofmax(P_(EMAX)−P_(PowerClass), 0) in dB. P_(EMAX) is the maximumtransmission power level a UE may use when transmitting on the uplinkcell in dBm and P_(PowerClass) is the maximum RF output of the UE in dBmin accordance with the UE power class. These values may be computed ormeasured on a per-subframe basis. Alternatively, these values arecomputed on another time granularity, such as by slot or radio frame. Inanother example, they are determined in irregular intervals. WhenQ_(rxlevmeas) cannot be defined, it may be set to minus infinity oranother value, such as an arbitrary low value.

Q_(D2Doffset) is transmitted by eNBs supporting D2D communications. Forexample, Q_(D2Doffset) is transmitted in a system information block(SIB) defining the parameters for D2D communication. When Q_(D2Doffset)is not received from the eNB, for example because the UE is out ofcoverage or the eNB does not transmit it, Q_(D2Doffset) is a fixed valuepre-configured in the UE, which may be specified, for example, by astandard. In one example, the default value of Q_(D2Doffset) is 3 dB.

A set of subframes S_(meas) is defined, where S_(meas) is the set ofsubframes on which ICT is measured. For example, S_(meas) may be the setof N consecutive subframes, where N is transmitted in a SIB orpre-configured when an SIB is not received. In another example, N ispredefined, for example defined by a standard. Alternatively, S_(meas)is any set of N subframes chosen by the UE within a time T, where N andT are transmitted in an SIB, pre-configured, or predefined.

The maximum ICT value over the set of subframes is:

ICT_(max)=max{ICT(s),sεS _(meas)}.

In one example, ICT_(max) is computed on a single subframe, andICT_(max)=ICT. The criterion to be in coverage may be:

ICT_(max)≧ICT_(Th),

where ICT_(Th) is a threshold which may be predefined, for example froma standard specification, broadcasted by the network in an SIB, or afixed predefined value. When:

ICT_(max)<ICT_(Th),

the UE is determined to be out-of-coverage.

The criterion:

ICT_(max)≧ICT_(Th)

is based on the fact that, when, during a given time, a single ICT valueis measured larger than the threshold, the device is consideredout-of-coverage. This provides extra protection for the cellularnetwork, and, to some extent, and when in doubt, determines that the UEis in coverage. In another example, another criteria is used, forexample, the minimum ICT value over the set of subframes is used, forexample:

ICT_(min)≧ICT_(Th),

where:

ICT_(min)=min{ICT(s),sεS _(meas)}.

In another example, the average ICT value over the set of subframes isused:

ICT_(avg)≧ICT_(Th),

where:

${{ICT}_{avg} = {\frac{1}{K}{\sum_{s \in S_{meas}}{{ICT}(s)}}}},$

and K is the number of subframes is S_(meas).

In additional examples, other criteria, such as the harmonic mean orgeometric mean, may be used.

In another example, two in-coverage thresholds, ICTs and ICTb are used.ICTs is used for obtaining the scheduling grants or only D2D broadcastresources, and ICTb is used only for discovery. In one example, ICTb ismore useful for idle UEs. ICTs and ICTb are calculated similarly to ICT,with different D2D offsets, where:

Q _(D2Doffset) _(—) _(B) Q _(D2Doffset) _(—) _(S),

and:

0≦Q _(D2Doffset) _(—) _(S).

The UE will not start registration to the cell even when it hassuccessfully detected a cell and decoded the master information block(MIB) and SIBs unless:

Srxlev=(Q _(rxlevmeas)−(Q _(rxlevmin) +Q_(rxlevminoffset))−Pcompensation)≧0.

A quality reference signal received quality (RSRQ) type threshold forin-coverage is:

INC_Qual=Q _(qualmeas)−(Q _(qualmin) +Q _(qualD2Doffset)).

However, the quality threshold may not be used when the serving cell ofthe D2D UE makes provisions for D2D resources, such as making themorthogonal to resources assigned to cellular information. The SIB mayindicate the value of Q_(qualD2Doffset). A default value ofQ_(qualD2Doffset) may be +∞. The UE may need to satisfy the S-Criteriato obtain D2D resource allocation commands from the eNB, unless thecommands are broadcast, for example in an SIB.

In one situation, an ICT or ICT_(max) may be greater than zero, but theUE only finds an acceptable cell where only emergency calls are allowed.In such a case, depending on the UE category, the UE may be consideredto be in or out of network from a D2D perspective, and use thecorresponding resource allocation method. For example, public safety UEsmay be considered to be in-network, while an ordinary UE would beconsidered to be out-of-network.

When the ICT or ICT_(max) is greater than zero, but the cell is a barredcell, D2D service may not be allowed until another cell is found, unlessthe UE is a public safety UE. When the UE is a public safety UE, acontention based scheme may be adopted. In another example, the cellprovides a pool of resources to UEs in a broadcast message, and the UEperforms contention-based transmission using the broadcast resourcepool.

When no RSRP is measured, the cell is out-of-coverage from a D2Dperspective. However, it may be in the coverage of another RAT. The UEis considered to have out-of-network coverage.

In another embodiment, out-of-network coverage is defined when the UE isin the any cell selection state, i.e. the UE is searching for a cell tocamp on, but cannot find one. In another embodiment, out-of-networkcoverage is defined when the UE was previously in connected mode andlost coverage. There may be some default resource or band the UE may usefor D2D in the any cell selection state, for example preprogrammed ordefined by the network during the last registration. However, this maybe limiting, for example to public safety uses. For example, in thecamped on any cell state, it is likely that a public safety UE shouldalso be allowed to use D2D on the default band or resource, or anotherband or resource indicated by the network in an SIB message.

The network may also provide a timer to the UE for how long it mayaccess these resources once it goes out of coverage. The eNB may updatethis out-of-coverage D2D resource allocation on a relatively dynamicbasis. For example, the eNB may eliminate it when there is no emergencyincident necessitating D2D communication, and the UE is in the campednormally state and the connected mode state.

In an embodiment, a UE assesses whether it is in-coverage or not usingan ICT computation. When the UE is in-coverage, it uses a first resourceassignment method, where the eNB schedules the D2D communication. Whenthe UE is out-of-coverage, the D2D UE obtains its resource allocationthrough a contention based resource allocation method.

FIG. 5 illustrates flowchart 190 for a method of D2D communication. TheUE decides D2D resource allocation using a timer. Initially, in step192, the UE requests resource allocation from an eNB while in-coverage.The UE connects to the eNB to request its D2D resource allocation.

Next, in step 194, the UE obtains the resource allocation and/or a timervalue from the eNB. In one embodiment, the UE reads the appropriate SIBand obtains the current value for out-of-coverage D2D resourceallocation and the timer value for this cell. The UE stores the resourceallocation and/or a timer value. When these values have changed, the UEupdates them.

Then, in step 196, the UE determines whether there is a loss of networkcoverage. When the UE loses coverage, it performs cellselection/reselection, and it is no longer camped on a cell. When thereis no loss of coverage, the UE returns to step 192 to request resourceallocation again. When the UE detects a loss of network coverage, itproceeds to step 198.

In step 198, the UE starts a timer. For example, the timer may beinitialized to the timer value stored from the last cell it camped onreceived in step 194. The timer counts down from the timer value. Inanother example, the timer value is predetermined.

In step 198, the UE uses the out-of-coverage resources from the cell itcamped on in step 194 for D2D communications. The UE continues to usethese resources while the timer has not expired and the UE is not campedon or connected to a cell.

In step 200, the UE determines whether it is camped on or connected to acell again. The UE may be camped on or connected to the same cell oranother cell. This may be the same cell it was previously camped on or atotally new cell. When the UE is camped on and connected to a cell, itproceeds to step 202. On the other hand, when the UE has still notcamped and connected to a cell, it proceeds to step 204.

In step 202, the UE stops the timer and returns to the in-coveragestate. The UE uses scheduling-based D2D resources from the eNB on whichit is camped. The UE returns to step 192 to request resource allocationwhile in coverage.

In step 204, the UE uses out-of coverage resources obtained from thelast cell it was connected to or camped on.

Then, in step 206, the UE determines whether the timer has expired. Whenthe timer has not expired, the UE returns to step 200 to determinewhether the UE is camped on or connected to a new cell. When the timerhas expired without camping on a new cell, the UE proceeds to step 208.

In step 208, the UE is out-of-coverage, for example in the any cellselection state. The UE stops using the out-of-coverage D2D resourcefrom the last camped cell, and reverts back to using the pre-configuredout-of-contact resource allocation. The UE may use contention-based D2Dresources.

FIG. 6 illustrates state diagram 210 with scheduling-based resourceallocation state 212 and contention-based resource allocation state 214.In scheduling-based resource allocation state 212, the UE is camped on acell, and performs scheduling-based resource allocation. When the UE isin scheduling-based resource allocation state 212, it computes g(ICT)and determines if g(ICT) is greater than zero. For example, g(ICT) maybe equal to ICT_(max)−ICT_(Th), ICT_(min)−ICT_(Th), orICT_(avg)−ICT_(Th). When g(ICT) is greater than zero, the UE remains inscheduling-based resource allocation state 212. On the other hand, wheng(ICT) is less than or equal to zero, the UE proceeds tocontention-based resource allocation state 214.

In contention-based resource allocation state 214, the UE is not campedon a cell, and performs contention-based resource allocation. The UEcalculates f(ICT) in contention-based resource allocation state 214, anddetermines whether f(ICT) is less than or equal to zero. When f(ICT) isless than or equal to zero, the UE remains in contention-based resourceallocation state 214. When f(ICT) is greater than zero, the UE proceedsto scheduling-based resource allocation state 212. In one example, andg(ICT)=f(ICT)+d, where d is an offset. The offset d is used to avoidrepeatedly switching back and forth between contention-based resourceallocation and scheduling-based resource allocation The UE remains inscheduling-based resource allocation state 212 as long as g(ICT)>0, evenwhen it cannot obtain scheduling information.

FIG. 7 illustrates resource allocation state diagram 220, which containscontention-based resource allocation with pre-configured resource poolstate 222, scheduling based resource allocation state 226, andcontention-based resource allocation with configurable resource poolstate 224. In contention-based resource allocation with pre-configuredresource pool state 222, the UE is not camped on a cell and has neverbeen in coverage. The UE performs contention-based resource allocationwith a pre-configured resource pool. The UE determines f(ICTb), whichmay be ICT_(max)−ICT_(Th), ICT_(min)−ICT_(Th), or ICT_(avg)−ICT_(Th).While f(ICTb) is less than zero, the UE remains in contention-basedresource allocation with pre-configured resource pool state 222. Whenf(ICTb) is greater or equal to zero, the UE proceeds to thescheduling-based resource allocation state 226.

In contention-based resource allocation with configurable resource poolstate 224, the UE has previously been camped on a cell, and has come outof coverage. The UE performs contention-based resource allocation with aconfigurable resource pool. The UE determines f(ICTs) and f(ICTb). Whenf(ICTs) is greater than zero, the UE proceeds to scheduling-basedresource allocation state 226. When f(ICTb) is greater than zero andf(ICTs) is less than or equal to zero, the UE remains incontention-based resource allocation with configurable resource poolstate 224. When f(ICTb) is less than zero, the UE proceeds to thecontention-based resource allocation with pre-configured resource poolstate 222.

In scheduling-based resource allocation state 226, the UE performsscheduling-based resource allocation while it is camped on a cell. TheUE calculates f(ICTb) and f(ICTs). When f(ICTb) is less than zero, theUE proceeds to contention-based resource allocation with pre-configuredresource pool state 222. When f(ICTs) is greater than zero, the UEremains in scheduling based resource allocation state 226. When f(ICTb)is greater than zero and f(ICTs) is less than or equal to zero, the UEproceeds to contention-based resource allocation with configurableresource pool state 224.

FIG. 8 illustrates flowchart 230 for an embodiment method of decidingbetween contention based D2D resource allocation or schedule-based D2Dresource allocation. ICT_(max), ICT_(min), ICT_(avg), or another ICTparameter may be used. Initially, in step 232, the UE begins theprocedure.

Next, in step 234, the UE obtains configuration information. Theconfiguration information is used to calculate the in-coverage criteria.In one example, the in-coverage criteria are specified in the thirdgeneration project protocol (3GPP) specification. Alternatively, thein-coverage criteria are manually preconfigured. In another example, thein-coverage criteria are conveyed to the UE by another means, forexample by receiving a message from a eNB, for example in the SIB. TheUE obtains the parameters for computing the ICT, as well as thethreshold for determining the resource allocation method(scheduling-based or contention-based).

After obtaining the configuration parameters, the UE computes the ICTparameters in step 236, such as ICT and ICT_(max), ICT_(min) and/orICT_(avg). In one example, ICT and ICT_(max) are computed periodically,so the UE bases its decision to use either a contention based orscheduling based resource allocation policy with current information.The periodicity may be known a priori or obtained in step 234.Alternatively, the computation is performed at irregular intervals. TheICT is:

ICT=Q _(rxlevmeas)−(Q _(rxlevmin) +Q _(D2Doffset))−Pcompensation.

ICT_(max) is:

ICT_(max)=max{ICT(s),sεS _(meas)}.

In one example, ICT_(max) is computed on a single subframe, andICT_(max)=ICT.

In step 238, the UE decides whether to use scheduling-based resourceallocation or contention based resource allocation based on the ICTparameters. In other examples, other variables, such as ICT, are used.For example, the UE may compare ICT_(max) to the ICT threshold. In oneexample, when:

ICT_(max)≧ICT_(Th),

the UE determines that the UE is in-coverage, and uses scheduling-basedresource allocation in step 244. On the other hand, when:

ICT_(max)<ICT_(Th),

the UE determines that it is out-of-coverage, and uses contention-basedresource allocation in step 240. In other examples, ICT_(min) orICT_(avg) is used instead of ICT_(max). ICT_(Th) is a threshold whichmay be predefined, for example from a standard specification,broadcasted by the network in an SIB, or a fixed predefined value.

In step 244, the UE performs scheduling-based resource allocation. TheUE requests D2D resources from the network, for example from the eNB.The network then grants a resource to the UE for D2D transmission. Inone example, the UE is in RRC_Connected mode, and may use the RACH.After step 244, the UE proceeds to step 246, and ends the procedure.

In step 240, the UE uses contention based resource allocation. Forexample, the UE may perform transmission with a contention-basedapproach relying on the CSMA protocol used in international electricaland electronics engineers (IEEE) 802.11, or another contention-basedmechanism. After step 240, the UE proceeds to step 242 and ends thisprocedure.

The UE may notify the receivers of whether it is using scheduling orcontention-based resource allocation. When the UE uses scheduledresources, and the allocation is performed semi-statically, it may bedesirable for the receivers to know, so they can monitor this set ofresources. For example, the receiver may want to avoid a discontinuousreception (DRX) like mode. The transmitting UE may broadcast ascheduling message to all potential receivers, where the transmitterindicates the resource allocation for the actual data transmission. Afield to indicate for how long this set of resources is valid for thistransmitter may also be indicated in the broadcast message.

In addition to the in-coverage case and out-of-coverage case, there maybe a partial coverage case. In one example of partial-coverage, the UEcannot communicate directly with the eNB, but is attempting tocommunicate with a UE which is in coverage. In one example, with partialcoverage, the resource allocation is contention-based. In anotherexample, contention-based resource allocation is used, but the resourcepool is pre-configured, with a priori knowledge by the two UEscommunicating D2D. This may reduce the UEs interfering with the eNB. Inan additional example, the resource allocation is scheduling-based, andthe in-coverage UE relays the grant to the out-of-coverage UE.

FIG. 9 illustrates flowchart 250 for a method of determining resourceallocation when scheduling-based resource allocation is used for partialcoverage. Initially, in step 252, the UE begins the procedure.

Then, in step 254, the UE obtains configuration information. Theconfiguration information is used to calculate the in-coverage criteria.In one example, the in-coverage criteria are specified in the thirdgeneration project protocol (3GPP) specification. Alternatively, thein-coverage criteria are manually preconfigured. In another example, thein-coverage criteria are conveyed to the UE by another means, forexample by receiving a message from a eNB, for example in the SIB. TheUE obtains the parameters for computing the ICT, as well as thethreshold for determining the resource allocation method(scheduling-based or contention-based).

Next, after obtaining the configuration parameters, the UE computes theICT parameters in step 256, such as ICT and ICT_(max), ICT_(min) and/orICT_(avg). In one example, ICT and ICT_(max) are computed periodically,so the UE bases its decision to use either a contention based orscheduling based resource allocation policy with current information.The periodicity may be known a priori or obtained in step 234.Alternatively, the computation is performed at irregular intervals. TheICT is:

ICT=Q _(rxlevmeas)−(Q _(rxlevmin) +Q _(D2Doffset))−Pcompensation.

ICT_(max) is:

ICT_(max)=max{ICT(s),sεS _(meas)}.

In one example, ICT_(max) is computed on a single subframe, andICT_(max)=ICT.

In step 258, the UE decides whether it is in coverage or out ofcoverage. In other examples, other variables, such as ICT, are used. Forexample, the UE may compare ICT_(max) to the ICT threshold. In oneexample, when:

ICT_(max)≧ICT_(Th),

the UE determines that the UE is in-coverage, and uses scheduling-basedresource allocation in step 244. On the other hand, when:

ICT_(max)<ICT_(Th),

the UE determines that it is out-of-coverage, and uses contention-basedresource allocation in step 240. In other examples, ICT_(min) orICT_(avg) is used instead of ICT_(max). ICT_(Th) is a threshold whichmay be predefined, for example from a standard specification,broadcasted by the network in an SIB, or a fixed predefined value.

In step 264, the UE determines whether the other UE with which it wantsto perform D2D communications is in coverage. For example, thisdetermination may be performed by using different discovery signals forout-of-coverage UEs. When the other UE is in coverage, the UE proceedsto step 260 to perform scheduling based resource allocation. On theother hand, when the other UE is also out of coverage, the UE proceedsto step 266 to perform contention based resource allocation.

In step 260, the UE performs scheduling-based resource allocation. Whenonly one UE is in coverage, the out-of-coverage UE may blindly rely ongrant requests and grant responses from and to the in-coverage UE, whichcommunicates directly with the eNB. In another example, theout-of-coverage UE decodes the grant request from the in-coverage UE,aggregates the grant response with its grant requests, and transmits asingle grant request to the eNB. Then resources are allocated to theout-of-coverage UE based on the received grant from the eNB. When bothUEs are in-coverage, both UEs directly communicate with the eNB. Afterstep 260, the UE proceeds to step 262, and ends the procedure.

In step 266, the UE uses contention based resource allocation. Forexample, the UE may perform transmission with a contention-basedapproach relying on the CSMA protocol used in international electricaland electronics engineers (IEEE) 802.11, or another contention-basedmechanism. After step 266, the UE proceeds to step 268 and ends thisprocedure.

In one example, an in-coverage UE may move to out-of-coverage or partialcoverage. The UE notifies the UE it is communicating with of its changeof status. In another example, an out-of-coverage UE moves toin-coverage. In this example, the UE stops the contention-basedtransmission to avoid interference on the cellular resources. The UEmoving into coverage immediately suspends the D2D communication andnotifies the communicating UE of its change of status.

Some techniques may be used to avoid rapidly switching back and forthbetween in-coverage and out-of-coverage. A hysteresis mechanism may beused with a different threshold for moving into coverage and moving outof coverage. In another example, when the UE has switched, it may beprevented from switching again for a given duration.

Because a UE moving into coverage may interfere with the eNB, thesemechanisms may only be used for switching out of coverage. For example,when a timer is used, the timer may be set to 0 seconds for switchingfrom in-coverage to out-of-coverage. When hysteresis is used, thethresholds may be such that there is no penalty for switchingout-of-coverage, so the hysteresis essentially only applies forout-of-coverage UEs.

FIG. 10 illustrates a block diagram of processing system 270 that may beused for implementing the devices and methods disclosed herein. Specificdevices may utilize all of the components shown, or only a subset of thecomponents, and levels of integration may vary from device to device.Furthermore, a device may contain multiple instances of a component,such as multiple processing units, processors, memories, transmitters,receivers, etc. The processing system may comprise a processing unitequipped with one or more input devices, such as a microphone, mouse,touchscreen, keypad, keyboard, and the like. Also, processing system 270may be equipped with one or more output devices, such as a speaker, aprinter, a display, and the like. The processing unit may includecentral processing unit (CPU) 274, memory 276, mass storage device 278,video adaptor 280, and I/O interface 288 connected to a bus.

The bus may be one or more of any type of several bus architecturesincluding a memory bus or memory controller, a peripheral bus, videobus, or the like. CPU 274 may comprise any type of electronic dataprocessor. Memory 276 may comprise any type of non-transitory systemmemory such as static random access memory (SRAM), dynamic random accessmemory (DRAM), synchronous DRAM (SDRAM), read-only memory (ROM), acombination thereof, or the like. In an embodiment, the memory mayinclude ROM for use at boot-up, and DRAM for program and data storagefor use while executing programs.

Mass storage device 278 may comprise any type of non-transitory storagedevice configured to store data, programs, and other information and tomake the data, programs, and other information accessible via the bus.Mass storage device 278 may comprise, for example, one or more of asolid state drive, hard disk drive, a magnetic disk drive, an opticaldisk drive, or the like.

Video adaptor 280 and I/O interface 288 provide interfaces to coupleexternal input and output devices to the processing unit. Asillustrated, examples of input and output devices include the displaycoupled to the video adapter and the mouse/keyboard/printer coupled tothe I/O interface. Other devices may be coupled to the processing unit,and additional or fewer interface cards may be utilized. For example, aserial interface card (not pictured) may be used to provide a serialinterface for a printer.

The processing unit also includes one or more network interface 284,which may comprise wired links, such as an Ethernet cable or the like,and/or wireless links to access nodes or different networks. Networkinterface 284 allows the processing unit to communicate with remoteunits via the networks. For example, the network interface may providewireless communication via one or more transmitters/transmit antennasand one or more receivers/receive antennas. In an embodiment, theprocessing unit is coupled to a local-area network or a wide-areanetwork for data processing and communications with remote devices, suchas other processing units, the Internet, remote storage facilities, orthe like.

While several embodiments have been provided in the present disclosure,it should be understood that the disclosed systems and methods might beembodied in many other specific forms without departing from the spiritor scope of the present disclosure. The present examples are to beconsidered as illustrative and not restrictive, and the intention is notto be limited to the details given herein. For example, the variouselements or components may be combined or integrated in another systemor certain features may be omitted, or not implemented.

In addition, techniques, systems, subsystems, and methods described andillustrated in the various embodiments as discrete or separate may becombined or integrated with other systems, modules, techniques, ormethods without departing from the scope of the present disclosure.Other items shown or discussed as coupled or directly coupled orcommunicating with each other may be indirectly coupled or communicatingthrough some interface, device, or intermediate component whetherelectrically, mechanically, or otherwise. Other examples of changes,substitutions, and alterations are ascertainable by one skilled in theart and could be made without departing from the spirit and scopedisclosed herein.

What is claimed is:
 1. A method for device-to-device (D2D)communication, the method comprising: determining that a first userequipment (UE) is out-of-coverage, the first UE having been previouslyin-coverage on a first cell; starting a timer upon determining that thefirst UE is out-of-coverage; determining whether the first UE hasreturned to be in-coverage after starting the timer; determining whetherthe timer has expired; and communicating, by the first UE directly witha second UE, using out-of-coverage resources from the first cell whenthe timer has not expired and the first UE has not returned to bein-coverage.
 2. The method of claim 1, further comprising communicating,by the first UE directly with the second UE, using contention-basedresource allocation when the timer has expired.
 3. The method of claim2, wherein the contention-based resource allocation comprises using arandom access channel (RACH).
 4. The method of claim 2, wherein thecontention-based resource allocation comprises using carrier sensingmultiple access (CSMA).
 5. The method of claim 1, further comprisingstopping the timer when the first UE has returned to be in-coverage. 6.The method of claim 1, further comprising communicating, by the first UEdirectly with the second UE, using in-coverage resources from a secondcell when the first UE has returned to be in-coverage on the secondcell.
 7. The method of claim 1, further comprising communicating, by thefirst UE directly with the second UE, using in-coverage resources fromthe first cell when the first UE has returned to be in-coverage on thefirst cell.
 8. The method of claim 1, further comprising: requesting, bythe first UE from the first cell, resource allocation while the first UEis in-coverage on the first cell; and receiving, by the first UE fromthe first cell, resource allocation information after requesting theresource allocation.
 9. The method of claim 8, wherein receiving theresource allocation comprises receiving the resource allocation on asystem information block (SIB).
 10. The method of claim 8, wherein theresource allocation information comprises in-coverage resourceallocation information.
 11. The method of claim 8, wherein the resourceallocation information comprises out-of-coverage resource allocationinformation.
 12. The method of claim 1, further comprising receiving, bythe first UE from the first cell, a timer value while the first UE isin-coverage on the first cell.
 13. The method of claim 1, whereinstarting the timer comprises: setting the timer to a timer value; andcounting down the timer from the timer value to zero.
 14. The method ofclaim 1, wherein determining that the first UE is out-of-coveragecomprises determining that the first UE is not camped normally.
 15. Themethod of claim 1, wherein determining whether the first UE has returnedto be in-coverage comprises determining whether the UE is campednormally.
 16. A method for device-to-device (D2D) communications, themethod comprising: computing an in-coverage threshold (ICT) parameter;determining whether to perform contention-based resource allocation orscheduling-based resource allocation in accordance with the ICTparameter; performing contention-based resource allocation to produceallocated resources when determining to perform contention-basedresource allocation; performing scheduling-based resource allocation toproduce the allocated resources when determining to performscheduling-based resource allocation; and communicating, by a first userequipment (UE) directly with a second UE using the allocated resources.17. The method of claim 16, wherein the ICT parameter is:Q _(rxlevmeas)−(Q _(rxlevmin) +Q _(D2Doffset))−Pcompensation, whereinQ_(rxlevmeas) is a measured reference service received power (RSRP),Q_(rxlevmin) is a minimum required reception level, Q_(D2Doffset) is anoffset, and Pcompensation is a maximum transmission power level in thefirst UE for uplink transmission minus a maximum radio frequency (RF)output of the first UE in accordance with a power class of the first UEand zero.
 18. The method of claim 16, wherein determining whether toperform contention-based resource allocation or scheduling-basedresource allocation comprises: determining an ICT threshold; determiningto perform scheduling-based resource allocation when the ICT parameteris greater than or equal to the ICT threshold; and determining toperform contention-based resource allocation when the ICT parameter isless than the ICT threshold.
 19. The method of claim 16, wherein the ICTparameter is selected from the group consisting of a maximum ICT valueover a set of subframes, a minimum ICT value over the set of subframes,and an average ICT value over the set of subframes.
 20. The method ofclaim 16 wherein determining whether to perform contention-basedresource allocation or scheduling-based resource allocation comprisesdetermining whether to perform contention-based resource allocation witha configurable resource pool, perform contention-based resourceallocation with a pre-configured resource pool, or performscheduling-based resource allocation.
 21. The method of claim 16,further comprising obtaining configuration parameters, wherein computingthe ICT parameter comprises computing the ICT parameter in accordancewith the configuration parameters.
 22. A first user equipment (UE)comprising: a processor; and a non-transitory computer readable storagemedium storing programming for execution by the processor, theprogramming including instructions to determine that the first UE isout-of-coverage, the first UE having been previously in-coverage on afirst cell, start a timer upon determining that the first UE isout-of-coverage, determine whether the first UE has returned to bein-coverage after starting the timer, determine whether the timer hasexpired, and communicate, directly with a second UE, usingout-of-coverage resources from the first cell when the timer has notexpired and the first UE has not returned to be in-coverage.